BIOENERGETICS I

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BIOENERGETICS I

• BC21D
• SEMESTER 1

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LECTURE OUTLINE

• Principles of thermodynamics
• First and second laws
• How ATP system operates

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INTRODUCTION

• Bioenergetics is the field of biochemistry
  concerned with the transformation and use of
  energy by living cells

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Energy

• Different forms of energy can be interconverted
• Chemical battery converts chemical energy into
  electrical energy
• During the interconversion, some energy is lost
  as heat i.e. the input is greater than the output

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First Law of Thermodynamics

• In any physical or chemical change the total
  amount of energy in the universe remains constant
• Energy cannot be created or destroyed

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Second Law of Thermodynamics

• All physical or chemical changes tend to proceed
  in such a direction that useful energy undergoes
  irreversible degradation into a randomized form
  called ENTROPY. They come to a stop at an
  equilibrium point, at which the entropy formed is
  the maximum possible under the existing conditions

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Useful Energy

• There are 2 types
• Free Energy energy that can do work at a
  constant temperature and pressure
• Heat Energy energy that can do work ONLY
  through a change in temperature
• Entropy is energy in a state of randomness it is
  unavailable useless energy

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• NEED TO IDENTIFY
• REACTING SYSTEM
• SURROUNDING

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Changes in Free Energy

• Changes in free energy, heat and entropy in
  chemical reactions at constant temperature and
  pressure are related to each other by
• ?G ?H T ?S
• ?G change in free energy
• ?Hchange in heat content or enthalpy
• ?Schange in entropy of the universe

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• When a reaction reaches equilibrium
• ?S is always ve (entropy of universe increases
• ?G of the reacting system is always ve
  (decrease in free energy of the system)
• ?H is defined by the amt of heat emitted or
  absorbed by the reacting system from the
  surroundings at CONSTANT TEMP and PRESSURE
• ?H is ve when heat is lost and
• ve when it is absorbed

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• CELLS ARE CHEMICAL ENGINES AND REQUIRE FREE
  ENERGY
• They get free energy from either energy-rich
  nutrient molecules or solar radiation

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Standard Free Energy

• Every chemical rxn has a standard free energy
  change ?G it is a constant of the rxn.
• ?G can be calculated from the equilibrium
  constant of the rxn under standard conditions
• Temp298K or 25C
• Press1 atm or 760mmHg

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• UNDER STANDARD CONDITIONS
• ALL Chemical reactions will tend to go in the
  direction resulting in a decrease in free energy
  of the system

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• ?G -ve -The reactants contain more free energy
  than the products FEASIBLE RXN will Proceed
• ?G ve -The reactants contain less free energy
  than the products NOT FEASIBLE rxn will tend to
  go in opposite direction

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Equilibrium Constant Keq

• aAbBlt----? cCdD
• Keq Cc Dd
• Aa Bb
• ?G -2.303 RT log Keq
• R gas constant1.987cal/mol.K
• T absolute temp 298K

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Relationship between Keq and ?G (under
standard conditions)
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• The standard free-energy change is different
  mathematical way of expressing its equilibrium
  constant
• pH 7 is by convention designated the standard pH
  of biological system
• ?G change in standard free-energy at pH 7.0
• ?G change in standard free energy
• ?G change in free-energy

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• ?G has characteristic values for different rxns
• Examples
• glucose-6-phosphate--?glucose-1-phosphate
• ?G-1.74kcal/mol
• ATPH2O-?ADP phosphate
• ?G-7.3kcal/mol

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Mathematical relationship between ?G and ?G

• For any reaction AB--gtCD
• ?G ?G2.303RT logCD
• AB

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Features of Standard Free Energy Values of a
Chemical RXN

• ?G of sequential rxns are additive
• A?B ?G1
• C?D ?G2
• A?C ?G1 ?G2
• Glucose-1-phosphate?glucose -6-phospate
  ?G-1.74Kcal/mol
• Glucose-6-phosphate?frutose-6-phosphate
  ?G0.40Kcal/mol
• Glucose-1-phosphate?fructose-6-phosphate
  ?G-1.34Kcal/mol
• A thermodynamically unfavourable rxn can be made
  favourable

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Examples in Biological Systems

• Activated Protein Conformation
• Activated Protein can store free energy and serve
  as energy conversion devices
• Myosin converts the phosphoryl potential of ATP
  into mechanical energy
• Active transport of Na and K across membranes
• Ionic Gradients across membranes
• Electrochemical potential of Na can be tapped to
  transport Ca2 out of cells and sugars into cells

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Adenosine Triphosphate (ATP)

• Universal currency of free energy in biological
  system
• Biological systems require energy for
• Mechanical work (muscle contraction)
• Active transport of molecules and ions
• The synthesis of macromolecules from simple
  precursors

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• Chemotrophs obtain energy from oxidation of
  food
• Phototrophs obtain energy from trapping light
  energy
• ATP is the free-energy donor in most energy
  requiring processes

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ATP

• ATP is a nucleotide consisting of an adenine, a
  ribose and a triphosphate unit
• Is the major link between energy-yielding and
  energy requiring cell activities
• During catabolism some of the free energy
  generated is utilised in the manufacture of ATP
  from ADP and Pi

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ATP History

• First discovered in muscle tissue by Karl Lohmann
  in Germany and almost simultaneously by Friske
  and Subbarrow in the US 1929
• First thought to be present in skeletal muscle
  tissue only, later found in all cells
• Fitz Lipmann (1941) postulated the concept of ATP
  as a universal carrier of chemical energy

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The Chemistry of ATP

• ATP, ADP,and AMP are nucleotides
• Nucleotides consist of
• a heterocyclic purine or pyrimidine base (purine
  in ATP)
• a 5-carbon sugar (D-ribose in ATP)
• 1or more phosphate groups
• In ATP the 2 terminal phosphate groups are high
  energy groups
• Std. free energy -7.3kcal/mol for ATP and ADP
  hydrolysis
• On hydrolysis the standard free energy change is
  -7.3 kcal/mol

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Other High energy Phosphate compounds

• Glucose-6-phosphate - 0.3kcal/mol
• Phosphoenol pyruvate -14.8kcal/mol
• 3-phosphoglycerol phosphate -11.8kcal/mol

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What makes ATP have a High ?G?

• Structural features of ATP
• There are 3 major structural determinants
• Degree of ionization of ATP and its hydrolysis
  products
• ATP4- H2O?ADP3- HPO42-H
• At pH 7 ATP is completely ionized to ATP4- ion
• On hydrolysis H is formed at a conc of 10-7 M
  this pulls the reaction to the right
• 2. At pH7 the molecules have 4 closely spaced ve
  charges which repel each other
• When ATP is hydrolysed one -ve phosphate ion is
  removed and releases some of the electrical
  stress pulling the reaction to the right

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What makes ATP have a High ?G?

• Structural features of ATP
• There are 3 major structural determinants
• ATP4- H2O?ADP3- HPO42-H
• Resonance Hybrids
• ADP3- HPO42 are resonance hybrids i.e. they
  possess much less energy in this configuration
  than in their original positions in ATP, thus
  pulling the rxn to the right
• The free energy released is as a result of the
  fact the products have a smaller free energy than
  the reactants

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ATP can act as an intermediate in
phosphate-transfer reactions

• Reactions of metabolism take place via a series
  of consecutive enzyme catalysed reactions linked
  by common intermediaries
• AB?CD DE?FG
• ATP functions as the energy carrying intermediate
  in the cell linking reactions requiring free
  energy with those delivering it
• Eg ATP d-glucose?ADP D-glucose -6-phosphate
  ADP

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ATP in muscle contractions

• Myosin (rodlike molecules in thick filaments of
  muscles) hydrolyse ATP by repetitivemake and
  break contacts eith thin filaments in such a way
  that a sliding force is exerted.
• Myosin and actin are specialized to transform
  chemical energy into mechanical energy for
  muscular contraction

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ATP in Active Transport across membranes
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ATP in bioluminescence

• In firefly and beetle the generation of a light
  flash requires that luciferin is activated by an
  enzymatic reaction with ATP in which
  pyrophosphate to form luciferyl adenylate which
  acts with O2 and luciferase to form oxyluciferin.
  This reaction is accompanied by the emission of
  light.